1. Field of the Invention
The present invention relates in general to a braking system for automotive vehicles such as four-wheel automobiles, two-wheel motor cycles or various vehicles or trucks for industrial purposes. More particularly, the invention is concerned with an anti-squeal braking system capable of preventing a brake squeal.
2. Discussion of the Prior Art
As a device for applying a brake to a vehicle, there are widely used friction brakes such as a disc brake or drum brake, wherein a friction member is forced against a rotating brake rotor, by a hydraulically operated actuator, so as to restrain the rotation of the brake rotor.
Such friction brakes tend to easily produce an unfavorable high-pitched noise, generally called "squeal" in the art, upon brake application. To prevent this squeal, various solutions have been proposed. For example, Laid-Open Publication 54-156079 (issued in 1979) of Japanese Utility Model Application discloses an anti-squeal shim which is interposed between a friction member and an actuator of a friction brake. Another typical solution to the sqealing of the friction brake is the use of a specially designed lining on a friction member, which exhibits friction characteristics suitable for avoiding the generation of a squeal. Both solutions indicated above are considerably effective to prevent the friction brakes from squealing.
However, the use of an anti-squeal shim or specially designed brake lining as presently available is not considered a completely satisfactory solution to the "squeal" problem of the friction brakes. In addition, the use of an anti-squeal shim may cause another inconvenience, that is, deteriorated operating feel of the brake, and increased possibility of a "brake drag" (a phenomenon in which the friction member remains in contact with the brake rotor after a fluid pressure is released from the actuator). It is understood that the use of a friction member having a comparatively low friction coefficient is effective to avoid the squealing of the friction brake. In this case, however, the braking effect per unit pressure of the actuator is lowered, whereby the force required to operate the actuator for providing a sufficient braking effect is accordingly increased. This means the need of using a large-capacity booster between the brake operating member such as a brake pedal, and a hydraulic pressure source such as a master cylinder to operate the brake actuator. Another drawback encountered in solving the squeal problem by using a specially designed friction member is derived from the fact that the friction coefficient of the friction member is varied with the operating temperature. Therefore, it is impossible that a specially designed friction member is effective to eliminate a brake squeal under any operating conditions of the friction brake.